Update to include several django-changes by Sunil and some url-changes by fonfon

I probably should have made this two commits, but here it is:

This pull request includes all updates from  NickDaly/Plinth that fbxat/Plinth missed so far, and some changes that I made in the meantime:

On the current version of  NickDaly/Plinth some URLs are broken when not using --server_dir=/plinth.
I adapted the URLs to not be created/constructed manually, but to use Django's named URLs and it's url-reverse functionality. Using them makes it easier to notice when URLs break, and to change large parts of the project's URLs without much trouble. Kind of best-practice in django (and other apps).

I did NOT yet change how URLs are handled within the main- and sub-menu, but I didn't want to do too many changes in the beginning.